1. Field of the Invention
The present invention relates to a method used in a wireless communication system and related communication device, and more particularly, to a method of data transmission timing control and discontinuous reception handling under joint Time Division Duplex (TDD) and Frequency Division Duplex (FDD) operation and related communication device.
2. Description of the Prior Art
Owing to mobile communication technology advancements in recent years, various communication services, such as voice call services, data transfer services, and video call services, etc., may be provided to users regardless of their locations. Most mobile communication systems are multiple access systems in which access and wireless network resources are allocated to multiple users. The multiple access technologies employed by the mobile communication systems include the 1× Code Division Multiple Access 2000 (1×CDMA 2000) technology, the 1× Evolution-Data Optimized (1×EVDO) technology, the Orthogonal Frequency Division Multiplexing (OFDM) technology, and the Long Term Evolution (LTE) technology. Evolved from the LTE technology, the LTE Advanced is a major enhancement of the LTE standard. The LTE Advanced should be compatible with LTE equipment, and should share frequency bands with the LTE communication system. One of the important LTE Advanced benefits is its ability to take advantage of advanced topology networks, wherein optimized heterogeneous networks have a mix of macros with low power nodes such as picocells, femtocells and new relay nodes.
In addition, carrier aggregation (CA) is supported in an LTE Advanced system to leverage unused or vacant resources for enhanced throughput performance. It essentially aggregates multiple component carriers (CC) at a user equipment (UE) so that multiple component carriers can be transmitted simultaneously. The CA can be applied to aggregate both contiguous and non-contiguous CCs. The network may configure the UE to aggregate a different number of CCs originating from a single evolved NodeB (eNB) or from different eNBs. In such a situation, the UE can simultaneously receive radio frequency (RF) signals via multiple CCs with single or multiple receivers and simultaneously transmit RF signals via multiple CCs with single or multiple transmitters. A record of configured set of CCs aggregated for signal and data transmission may be maintained by both the UE and the eNB for keeping information regarding the configured CCs for the UE.
In the 3GPP Rel-10 specification, the aggregated component carriers should operate under the same duplexing mode. In other words, they should all belong to either time division duplex (TDD) CC or frequency division duplex (FDD) CC. This limitation reduces effort on the design of the whole CA mechanism. Yet, in practical wireless systems there is a need for leveraging unused or vacant component carriers which share different duplexing mode to the current used component carriers. However, if FDD and TDD component carriers are aggregated jointly, the physical uplink control channel (PUCCH), e.g. hybrid automatic repeat request (HARQ) acknowledgement (ACK) or negative-acknowledgement (NACK), feedback timing should be redesigned from legacy systems because FDD does not fall into any TDD configurations.
In the prior art, the PUCCH is transmitted on a primary cell (a PCell, or a primary carrier) while a downlink transmission (e.g. PDSCH transmission) may happen on the primary carrier or secondary cells (SCells, or secondary carriers). The associated feedback of the downlink transmission in a serving cell (which may be the primary cell or other carriers in CA) cannot be fed back on the primary cell by directly following the serving cell feedback timing. Otherwise, the UE might be forced to have PUCCH transmission within a downlink subframe on the primary cell and cause severe interference or system errors. In some cases, the UE may be forced to drop the PUCCH.
Moreover, when an FDD CC and a TDD CC are aggregated to transmit to the UE, it is not clear in the prior art how long a HARQ round trip time (RTT) Timer should be set by the UE. Inappropriate setting of the HARQ RTT Timer could cause certain issues to the communication system or increase the system delay time. For example, when a PCell of the UE is on the TDD CC, the UE may need to transmit HARQ feedbacks (ACK or NACK) on a PUCCH to the PCell for reception of downlink data on an FDD CC. In this case, 8 milliseconds may not be long enough for the HARQ RTT timer of the downlink HARQ process for reception of data on the FDD CC, so the UE may not monitor the PDCCH for a retransmission corresponding to the downlink HARQ process after the HARQ RTT timer and drx-RetransmissionTimer expire. Therefore, the UE cannot receive a retransmission if the retransmission is transmitted after the HARQ RTT timer and drx-RetransmissionTimer expire. On the other hand, setting the HARQ RTT timer to a longer value would cause the receiver of the UE to keep awake unnecessarily. In such a situation, the UE does not switch its receiver to a sleep mode, which therefore wastes the battery power of the UE.
Therefore, how to handle data transmission and enhance operation of discontinuous reception (DRX) under joint TDD-FDD operation in a wireless communication system is an important topic to be addressed and discussed.